"Formose" is a term used to define the known mixtures of low molecular weight polyhydroxyl compounds (polyhydric alcohols, hydroxyaldehydes and hydroxyketones) which are formed by the condensation of formaldehyde hydrate.
The preparation of mixtures of polyhydric alcohols, hydroxyaldehydes and hydroxyketones by the autocondensation of formaldehyde hydrate has been described in several publications in the literature. The following are examples of relevant literature references: Butlerow and Loew, Annalen 120, 295 (1861); J.pr.Chem, 33, 321 (1886); Pfiel, chemische Berichte 84, 229 (1951); Pfeil and Schroth, chemische Berichte 85, 303 (1952); R. D. Partridge and A. H. Weiss, Carbohydrate Research 24, 29-44 (1972); the formoses of glyceraldehyde and dihydroxyacetone according to Emil Fischer; German Pat. Nos. 822,835, 830,951 and 884,794; U.S. Pat. Nos. 2,224,910; 2,269,935; and 2,272,378 and British Pat. No. 513,708. These known processes, however, involve certain disadvantages such as toxicologically harmful catalysts, low volume/time yields and discolored by-products. New processes by which formoses which are virtually colorless and free from undesirable by-products can be obtained in high yields with the aid of conventional catalysts have recently been developed.
According to one of these new processes, the condensation of formaldehyde hydrate is carried out in the presence of a catalyst consisting of a soluble or insoluble lead (II) salt, or of lead (II) ions bound to a high molecular weight carrier, and in the presence of a cocatalyst consisting of a mixture of hydroxyaldehydes and hydroxyketones of the kind which is obtained from the condensation of formaldehyde hydrate and which is characterized by the following molar ratios:
______________________________________ Compounds with 3 carbon atoms/ compounds with 4 carbon atoms: 0.5:1 to 2.0:1; Compounds with 4 carbon atoms/ compounds with 5 carbon atoms: 0.2:1 to 2.0:1; Compounds with 5 carbon atoms/ compounds with 6 carbon atoms: 0.5:1 to 5.0:1. ______________________________________
In these mixtures the proportion of components with 3 to 6 carbon atoms is at least 75% by weight, preferably more than 85% by weight, based on the whole cocatalyst. The reaction temperature used is generally between 70.degree. and 110.degree. C., preferably between 80.degree. and 100.degree. C., and the pH of the reaction solution is adjusted by controlled addition of a base, first to a value of from 6.0 to 8.0, preferably from 6.5 to 7.0, until 10 to 60%, preferably 30 to 50% conversion has been reached, and thereafter to a value of from 4.0 to 6.0, preferably from 5.0 to 6.0. It has surprisingly been found that the proportion of components in the mixtures of polyols, hydroxyaldehydes and hydroxyketones obtained could be varied in a reproducible manner by this special method of pH control followed by cooling at different residual formaldehyde contents of 0 to 10% by weight, preferably 0.5 to 6% by weight.
When the autocondensation of formaldehyde hydrate has been stopped by cooling and/or by inactivation of the lead catalyst with acids, the catalyst may be removed in known manner and the water contained in the product is evaporated off. Further details of this process may be found in German Offenlegungsschrift No. 2,639,084.
According to another method of preparing highly concentrated, colorless formoses in high volume/time yields, aqueous formalin solutions and/or para-formaldehyde dispersions are condensed in the presence of a soluble or insoluble metal catalyst and in the presence of a cocatalyst which has been obtained by the partial oxidation of a dihydric or polyhydric alcohol with a molecular weight of 62 to 242 and containing at least two vicinal hydroxyl groups, or a mixture of such alcohols. During this reaction the pH of the reaction solution is maintained between 6.0 and 9.0 by controlled addition of a base up to 5 to 40% conversion. The pH of the reaction mixture is thereafter adjusted to 4.5 to 8.0 until termination of the condensation reaction. During this second reaction phase, the pH is 1.0 to 2.0 units lower than during the first reaction phase. The reaction is then stopped at a residual formaldehyde content of from 0 to 10% by weight by inactivation of the catalyst. The catalyst is then removed. This method has been described in German Offenlegungsschrift No. 2,718,084.
High grade formoses can also be obtained by the condensation of formaldehyde in the presence of a metal catalyst and more than 10% by weight, based on the formaldehyde, of one or more dihydric or polyhydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds. Such formose-polyol mixtures are the subject matter of German Offenlegungsschrift No. 2,714,104.
It is particularly economical to produce formose directly from synthesis gases containing formaldehyde, i.e. without first preparing aqueous formalin solutions or paraformaldehyde. The synthesis gases obtained from the large scale industrial production of formaldehyde are introduced continuously or discontinuously at temperatures of between 10.degree. and 150.degree. C. into an absorption liquid consisting of water, monohydric or polyhydric low molecular weight alcohols, and/or higher molecular weight polyhydroxyl compounds. Compounds capable of enediol formation may be used as cocatalysts along with soluble or insoluble metal compounds as catalysts, optionally bound to high molecular weight carriers. The absorption liquid is at a pH of 3 to 10. The formaldehyde is condensed in situ in the absorption liquid optionally also in a reaction tube or cascade of stirrer vessels following the vessel containing the absorption liquid. Autocondensation of formaldehyde is stopped at a residual formaldehyde content in the reaction mixture of from 0 to 10% by weight by cooling and/or by inactivation of the catalyst with acids, and the catalyst is finally removed. For further details of this process, see German Offenlegungsschriften Nos. 2,721,093 and 2,721,186.
Formoses prepared in this way may also be subsequently converted into their hemiacetals with excess formaldehyde or .alpha.-methylolated by reaction with formaldehyde in the presence of bases.
The properties of the formoses can be varied within wide limits by controlling the formaldehyde condensation process. In general, the further the condensation reaction continues, i.e. the lower the residual formaldehyde content, the higher is the average molecular weight obtained and hence the hydroxyl functionality of the formoses. If the reaction is continued to a residual formaldehyde content of from 0 to 1.5% by weight, a formose containing approximately 25% by weight of components with 5 carbon atoms, 45% by weight of compounds with 6 carbon atoms and approximately 20% by weight of compounds with 7 or more carbon atoms is obtained. At the same time, a total of only about 10% of polyols, hydroxyketones and hydroxyaldehydes with 2, 3 and 4 carbon atoms is obtained. This corresponds to an average hydroxyl functionality of approximately 5.
Different proportions of components are obtained by stopping the formaldehyde condensation at somewhat higher residual formaldehyde contents. When the condensation reaction is stopped at a formaldehyde content of from about 2 to 2.5%, a mixture of polyvalent alcohols, hydroxyaldehydes and hydroxyketones having an average hydroxyl functionality of approximately 4 is obtained. Yet other proportions of components with a substantially lower average hydroxyl functionality are obtained when the condensation reaction is stopped at residual formaldehyde contents higher than 2.5.
In all of these processes, crossed Cannizzaro reactions between formaldehyde and the carbonyl groups of the formaldehyde condensation products take place to a considerable extent simultaneously with the autocondensation of formaldehyde hydrate. The formoses obtained contain a considerable quantity of polyhydric alcohols in addition to hydroxyaldehydes and hydroxyketones. This may be advantageous for many purposes, particularly when it is intended to hydrogenate the formoses into mixtures of polyhydric alcohols. For other purposes, however, for example for the production of highly flame-resistant polyurethane foams or the use of the formoses as substrates in the nutrient media of microorganisms, it is advantageous to obtain a formose containing as high a proportion of reducing sugars as possible, i.e. as little as possible of polyhydric alcohols formed by crossed Cannizzaro reactions. Moreover, the organic acids formed in the crossed Cannizzaro reaction (mainly formic acid with traces of acetic, lactic, glycollic and saccharic acids), are undesirable for many potential applications. They can only be removed from the reaction mixture by a relatively complicated procedure using anion exchangers.